Process for converting methanol to a high-methane gas

ABSTRACT

Methanol is converted into a high-methane gas by cracking methanol vapor in one or more stages under super-atmospheric pressure at elevated temperatures in contact with a catalyst and preferably in the presence of water vapor. A methane containing gas having a higher methane concentration than the gas produced by the cracking of methanol alone is added to the feedstock to be converted prior to the cracking reaction.

United States Patent [191 Moller et al.

1 Aug. 5, 1975 1 1 PROCESS FOR CONVERTING METHANOL TO A HlGl-l-METHANE GAS [75] Inventors: Friedrich Wilhelm Moller,

Friedrichsdorf; Heinz .lockel, Klein-Gerau; Karl Bratzler, Bad Homburg; Hans-Joachim Renner, Frankfurt am Main. all of Germany [73] Assignec: Metallgesellschaft Aktiengesellschaft,

Frankfurt am Main, Germany [22] Filed: Aug. 1, 1974 [21] App]. No.: 493,997

[30] Foreign Application Priority Data Aug. 16 1973 Germany 1. 2341288 [52] US. Cl. 48/197 R; 48/214; 260/449 M [51] Int. Cl. C0lb 2/14; C07c 1/20 [58] Field of Search 48/214, 197 R. 215; 260/449 M; 252/373 [56] References Cited UNlTED STATES PATENTS 3.621665 12/1971 Thompson 48/197 R men CH4 GAS METHANOL CRACKING REACTOR OTHER PUBLICATIONS Royal, Why not Methanol as SNG Feedstock?," Pipeline and Gas Journal, February, 1973, pp. 5862.

Primary Examiner-R. E. Serwin Attorney. Agent, or Firm-Burgess, Dinklage & Sprung [57] ABSTRACT Methanol is converted into a highmethane gas by cracking methanol vapor in one or more stages under super-atmospheric pressure at elevated temperatures in contact with a catalyst and preferably in the presence of water vapor. A methane containing gas having a higher methane concentration than the gas produced by the cracking of methanol alone is added to the feedstock to be converted prior to the cracking reaction.

12 Claims, 3 Drawing Figures SCRUBBER PATENTED AUG 51975 SHEET 1 6F 3 gal mwmmnmum ou PATENTED 19% PROCESS FOR CONVERTING METHANOL TO A HlGH-METHANE GAS BACKGROUND This invention relates to a process of converting methanol into a high-methane gas by a cracking of methanol vapor in one or more stages under superatmospheric pressure and at elevated temperatures in contact with a catalyst, preferably in the presence of water vapor.

The requirement for natural gas as fuel gas is steadily increasing. This requirement and the partly simultaneous decrease of the availability of natural gas in the countries concerned results in a compelling need to provide natural gas from other sources.

An increasingly developed method of providing natural gas involves the transportation of liquefied natural gas by ship from countries having large resources to the consuming countries. The increasing use of this transportation is opposed by the fact that methane involves a safety hazard because it is easily inflammable. For this reason it has already been proposed to crack the natural gas with water vapor so as to form a mixture of carbon monoxide and hydrogen, at the place where the natural gas is produced, and to convert the mixture by a catalytic reaction into methanol, which is then transported by ship. At the place of consumption, the methanol is burnt or reconverted into methane.

The methanol can be converted to methane by an addition of water vapor in contact with a catalyst at elevated temperatures and under superatmospheric pressure. The catalyst consists suitably of a nickel catalyst, which contains nickel on a support material.

The catalytic cracking of methanol is effected in accordance with the reaction equation 4 CH OH 3 CH CO 2H O and involves a heat effect A H (400C.) of l7.4 kilocalories per carbon atom.

The large evolution of heat involved in the cracking of methanol necessitates a sufficiently rapid dissipation of heat in order to avoid an excessive temperature rise of the catalyst bed. A temperature rise beyond 500 C. will inevitably deteriorate the catalyst.

It is known to avoid or at least limit these difficulties in that the gas mixture leaving the reactor is sufficiently cooled and part of the gas is recycled and is reintroduced into the reactor together with the reaction mixture (Pipeline and Gas Journal, Feb. 1973, pages 59-61). The recycled gas then acts as a diluent and owing to its specific heat as a heat absorber in the reactor. To enable such heat absorption to become effective, large quantities of gas are required so that the gas recycle rate must be a multiple of the product gas rate. For instance, when it is desired to avoid an excessive rise of the reaction temperature by the highly exothermic reaction by which methanol is converted to methane, the recycle gas rate must be times the product gas rate. This involves a high expenditure as regards equipment and energy because large recirculating pumps must be installed, which consume much energy and require maintenance. Besides, the recycle gas must be compressed at elevated temperature in order to avoid a condensation of vapor.

SUMMARY; Surprisingly it has been found that the evolution of heat by the cracking of methanol may be limited in a simple manner. This is accomplished according to the invention in that a methane-containing gas which has a higher methane concentration than the gas produced by the cracking of methanol alone is added to the feed- (fl-I H O CO+ 3H lt has a heat of formation A H (400C.) of +49 kilocalories per carbon atom.

According to the above-mentioned equation, the gas produced by the cracking of methanol alone, consists theoretically of by volume of methane, on a dry basis. The recycling of a large stream of cooled, moist product gas in accordance with the state of the art results only in a dilution and in a reduction in output rather than in a decrease of the thermal loading of the cracking reactor. In the known process the product gas has already the composition which corresponds to the equilibrium of the cracking reaction so that it behaves in the methanol-cracking reactor like a non-reacting inert gas, which cannot participate in the reaction. If a surplus of methane is provided, the cracking thereof will consume all or part of the heat which is evolved by the reaction of methanol.

It is also of great importance for the process according to the invention that the high-methane gas fed into the methanol-cracking reactor is produced as an intermediate or end product in the methanol-converting process. For this reason a gas from an external methane source is not required. In the production of a highmethane gas, which preferably can be a substitute for natural gas, by a reaction of methanol, the catalytic cracking results first in a product gas, which must be subjected to a catalytic methanization so that it can be supplied to the consumer supply system. By the methanization, the residual carbon monoxide and part of the carbon dioxide are catalytically hydrogenated until virtually all hydrogen still contained in the product gas has been consumed. The remaining carbon dioxide is then scrubbed off and the resulting product gas containes one-third more methane than the product gas coming directly from the cracking stage. When the high-methane product gas is recycled into the cracking reactor, the catalytic thermal treatment in the presence of water vapor causes the high-methane product gas to be cracked back to the equlibrium composition present in the reactor so that its methane content is reduced by about one-fourth. This cracking of methane consumes a considerable quantity of heat, which is evolved by the exothermal cracking of methanol.

The conversion of methanol to a high-methane gas is accomplished in one or more reactors under a pressure of 5-l 00, preferably 15-60, kilograms per square centimeter absolute pressure. The temperature is in the range of approximately 250550 C. and in most cases spinel.

It has been found that the size of the nickel crystallites in the support is important for an optimum behaviour of the catalyst. In accordance therewith, the nickel crystallites in the novel catalyst should have a particle size below about 300 Angstroms and a most frequent particle size in the range of 50-150 A. In a catalyst which has been used for 2000 hours, the size of the largest nickel crystallites should be below 400 A and the most frequent particle size should have shifted only into the range of 80-200 A. The hydrogen adsorption of a novel catalyst of the preferred kind should exceed 3 milliliters per gram. Catalysts having the abovemen tioned properties distinguish by particularly good conversion results and above-average lives.

DESCRIPTION OF THE DRAWING Examples of different modes of carrying out the process will now be explained with reference to the drawing, in which FIG. 1 is a flow diagram for cracking methanol in the presence of a high-methane gas according to the invention;

FIG. 2 is a flow diagram for cracking methanol in two stages according to the invention; and

FIG. 3 is a flow diagram of a methanol-cracking plan having a cooler-saturator system.

DESCRIPTION In the process shown in FIG. 1, a mixture of methanol vapor from line M and water vapor from line D and high-methane gas from conduit 2 are jointly fed into the reactor through conduit 1. The feed mixture is at a temperature of about 250400 C. The reactor 10 contains a nickel catalyst of the kind described hereinbefore.

In the reactor 10, the methanol is cracked to form methane, carbon dioxide, and water. At the same time, part of the also fed methane of the high-methane gas reacts with the present water vapor to form carbon monoxide and hydrogen.

The cracking methane is highly endothermic and the cracking of methanol is exothermic. The quantities of simultaneously cracked methane and methanol are adjusted so that the heat is consumed and evolved in approximately equal quantities. This prevents an excessive temperature rise in the reactor. A certain surplus of water vapor above the theoretical requirement is necessary to prevent a formation of carbon black by the Boudouard reaction, which would considerably reduce the life of the catalyst. For this purpose, however the weight ratio of the fresh water vapor added from the outside to the methanol feed need not exceed 4:1 and is usually below 2:l. It is important that the gas mixture flowing over the catalyst has a weight ratio not below 0.7:l of water vapor to methanol.

The product gas from the reactor 10 contains mainly methane and also contains carbon oxides, hydrogen, and water vapor. It is cooled in a heat exchanger 11 to temperatures of about 220350 C. and is subsequently subjected to a methanation, which is effected in a basically known manner in contact with a catalyst, such as a nickel catalyst.

According to FIG. 1, the methanation is effected in two stages although a methanation in a single stage may also be adopted. The gas to be methanized flows in conduit l2 and then through a first methanation reactor 20, in which a wet methanation is effected, before which the gas mixture has not been predried. The product gas from the reactor is then precooled in a heat exchanger 21 and subsequently passed through a cooler 22, in which condensed water is separated. The

gas which has thus been dried and cooled is supplied in conduit 23 to a heat exchanger 25, in which it is reheated by hot product gas from the reactor 24, and is passed through a second methanation reactor 24.

The gas leaving the reactor 24 contains almost only methane and, in addition thereto, mainly some carbon dioxide. When the product gas has been cooled in the heat exchangers 25 and 29, the CO is removed almost completely in the subsequent, conventional, carbon dioxide scrubber 30 by a treatment with, e.g., hot potash solution. The gas leaving the CO scrubber 30 contains usually more than methane by volume and can be substituted for natural gas (SNG). A partial stream of that gas which consists almost exclusively of methane and which has been cooled in 40 is branched off in conduit 2. The branched-ofi' gas is boosted by the blower 41 and then heated in the heat exchanger 21 before it is recycled to the conduit 1, in which it is added to new feedstock consisting of methanol vapor and water vapor.

FIG. 2 shows a process comprising two methanolgasifying stages. About 50-75% of the methanol feedstock M flowing in conduit 3 together with the steam D and about three-fourths of the high-methane gas recycled in conduit 2 are mixed in conduit 1 and fed to the first conversion reactor 10. The product gas is subjected to interstage cooling in the heat exchanger 11 and is fed through conduit 12 into the feed conduit 6 of a second methanol-gasifying reactor 15. The mixture fed into the reactor 15 consists of the product gas from the reactor 10, the remainder of the methanol feedstock from conduit 4, and the remainder of the recycled highmethanol gas from conduit 7. The reaction conditions in reactor 10 and 15 are the same as those described in connection with FIG. 1.

The product gas from the reactor 15 is cooledin the heat exchanger and first subjected to a wet methanation in the reactor 20. This is followed by a precooling in the heat exchanger 21 and a main cooling in 22 with a separation of condensed water. When the gas has been heated in the heat exchanger 25, the gas is supplied in conduit 28 to a reactor 24 for a dry methanation. The subsequent treatment in the CO scrubber 30 for producing the gas which can be used as a substitute for natural gas (SNG) and the recycling of a partial stream in conduit 2 have already been described with reference to FIG. 1.

A modification of the process shown in FIG. 3 uses a methanol-cracking reactor 10, which is fed with a mixture of methanol vapor M and high-methane gas, which has been recycled in conduit 2 and still contains water vapor. The product gas is cooled in the heat exchanger 11 and then fed to a reactor 20 for a wet methanation and through the heat exchanger 21 is fed into a cooler 28, in which the gas is cooled by relatively cold trickling water, whereby most of the water vapor contained in the gas is condensed. The cooling water is thus heated and is then re-used at anotherpoint of the process, in a saturator 36. The water conduits are omitted in FIG. 3, for the sake of clearness. The dried gas is then passed through a C0 scrubber 30.

A partial stream of the gas leaving the CO scrubber is compressed in the boosting blower or compressor 35 and is then fed into a saturator 36, in which the gas is saturated with water vapor from trickling hot water supplied from cooler 28. The high-methane gas which is saturated with water vapor is passed in conduit 2 and heated in the heat exchanger 21 and then fed back into the reactor 10. 1

The other part of the gas from the CO scrubber 30 is passed through a conduit 31 and a heat exchanger 37 and then subjected to dry methanation in the reactor 24. The product gas is cooled in the heat exchanger 37 and is then available as a substitute for natural gas (SNG).

The compressor 35 is driven by a steam turbine 39. The heat content for the superheated steam used to drive the turbine is extracted in the heat exchanger 11 from the product gas discharged from the reactor 10. Alternatively, the process shown in FIG. 3 may be carried out in an economical manner without a utilization of waste heat in the turbine 39 for driving the compressor 35.

EXAMPLE 1 Table I A B C D CO by vol. 1.0 11.8 below 0.1 9.9 CO, by vol. 0.3 10.3 H by vol. 1 0 15.0 3.6 0.9 CH by vol. 98.0 72.9 86.1 89.2

Steam at 310 C., at a rate of 20 kilograms per hour,

and vaporized methanol, at a rate of kilograms per hour, are added to the recycle gas and admixed in conduit 1. The mixture is passed in reactor 10 under a pressure of 45 kilograms per square centimeter (absolute pressure) in contact with a catalyst which contains 40% by weight nickel in an MgO-SiO support. As a result of the exothermic methanol-gasifying reaction, the gas at the outlet of the reactor 10 has a temperature of 480 C. Moist cracked gas which has the composition stated in column B of Table I becomes available at a rate of 49.0 standard cubic meters per hour (22 cubic meters per hour on a dry basis). The cracked gas still contains 1.2 standard cubic meters water vapor per standard cubic meter of dry gas.

The temperature of this cracked gas is reduced to 300 C. in the heat exchanger 11 and the cracked gas is then methanized in the reactor 20, which is identical to the reactor 10. As a result of the exothermic methanation reaction, the gas is heated to 360 C. Moist methanized cracked gas having the composition stated in column C of Table I is produced at a rate of 47.7 standard cubic meters per hour 19.6 standard cubic meters per hour on a dry basis). The gas still contains 1.43 standard cubic meters of water vapor per standard cubic meter of dry gas. When this gas has been precooled in 21 and water vapor has been condensed therefrom in 22, the gas is remethanized in another reaction stage 24 at a temperature of about 320 C., whereby gas having the composition stated in column D of Table I is produced at a rate of 19.1 standard cubic meters per hour.

In a C0 scrubber 30, CO at a rate of 1.7 standard cubic meters per hour is scrubbed from this gas so that a high-methane product gas (SNG) having the composition stated in column A of Table I is produced at a rate of 17.4 standard cubic meters per hour. Part of this gas, which can be substituted for natural gas, is recycled at a rate of 12 standard cubic meters per hour in conduit 2 to the methanol gasifier. The remainder is withdrawn from the system as a product gas.

EXAMPLE 2 Table II A B c 00,, b vol. 10 11.8 13.4 C0, by vol. 0.3 0.3 1-1,,% b vol. 1.0 15.0 11.6 c1-1.,, by vol. 98.0 72.9 74.7

Part of the recycle gas in conduit 2, at a rate of 12 standard cubic meters per hour, i.e. are fed into the feed conduit 1 leading to the first gasification reactor 10. Superheated steam at a rate of 20 kilograms per hour and vaporized methanol at a rate of 10 kilograms per hour are added from conduit 3 to this gas. The mixture has a temperature of 310 C.

Under a pressure of 45 kilograms per square centimeters (absolute pressure) this reaction mixture is passed in reactor 10 in contanct with a catalyst which contains 50% by weight nickel on an MgO-Al O support. As a result of the exothermic gasification reaction, the temperature rises up to 480 C.

Moist cracked gas having the composition stated in column B of Table II is produced at a rate of 49.0 standard cubic meters per hour (22.2 standard cubic meters per hour on a dry basis). This cracked gas still contains 1.2 standard cubic meter water vapor per standard cubic meter of dry gas. The temperature of the cracked gas is reduced to 310 C. in 11. Thereafter, the remaining high-methane recycle gas from conduit 7, at a rate of 4 standard cubic meters per hour, and vaporized methanol from conduit 4, at a rate of 10 kilograms per hour, are admixed to the cracked gas. The resulting reaction mixture is reacted in a second gasification stage 15.

Moist dry cracked gas a temperature of 480 C. leaves the reactor 15 at a rate of 63.8 standard cubic meters per hour (33.8 standard cubic meters on a dry basis). This gas has the composition statedin column C of Table II. The gas still contains 0.89 standard cubic meter of water vapor per standard cubic meter of dry gas.

This cracked gas is cooled to 300 C. in 14 and then just as in Example 1 is subjected to a wet methanation in reactor 20 and to a dry methanation in reactor 24. Identical catalyst materials are used in reactors 10, 15, 20, and 24. When CO has been scrubbed off at a rate of 3.4 standard cubic meters per hour, the gas which can be substituted for natural gas and has the composi- 7 tion stated in Column A of Table II becomes available at a rate of 26.8 standard cubic meters per hour. Part of this gas, at a rate of 16.0 standard cubic meters per hour, is recycled in conduit 2. The remainder is discharged as product gas.

EXAMPLE 3 In a processd as illustrated in FIG. 3, a moist gas having the composition stated in column A of Table III is recycled to the reactor in conduit 2 at a rate of 61.3 standard cubic meters per hour (31.0 standard cubic meters per hour on a dry basis).

The gas still contains 0.98 standard cubic meter water vapor per standard cubic meter of dry gas. This recycle gas, together with vaporized methanol at a rate of 20 kilograms per hour, is gasified in the reactor 10 at a temperature of 300 C. and under a pressure of 45 kilograms per square centimeter (absolute pressure) in contact with a catalyst which contains 50% by weight nickel on a support of MgO-Al O Moist cracked gas which is at a temperature of 500 C. and has the composition stated in column B of Table III is thus produced at a rate of 85.1 standard cubic meters per hour (50.4 standard cubic meters per hour on a dry basis). The cracked gas still contains 0.67 standard cubic meter of water vapor per standard cubic meter of dry gas.

The temperature of this cracked gas is reduced to 275 C. in the heat exchanger 1 1. The gas is then methanized in the reactor 20 in contact with the catalyst which isalso used in the reactor 10. As a result of the exothermic methanizing reaction, the gas mixture is heated to 340 C. Moist methanized cracked gas which is at 340 C. and has the temperature stated in column C of Table III is produced at a rate of 82.5 standard cubic meters per hour (45.3 standard cubic meters per hour on a dry basis). The gas still contains 0.82 standard cubic meter water vapor per standard cubic meter of dry gas.

In the cooler 28, the gas is contacted with relatively cold trickling water to condense the water vapor content of the gas whereby the cooling water is heated. The gas emerging form the cooler 28 is then treated in the CO scrubber 30, in which CO is removed at a rate of 3.4 standard cubic meters per hour. Dry methanized cracked gas having the composition stated in column A of Table III becomes available a rate of 41.9 standard cubic meters per hour. Part of this high-methane gas, at a rate of 10.9 standard cubic meters per hour, is withdrawn in conduit 3 l as a product gas and in reactor 24 is subjected to a dry final methanation to increase its calorific value.

The remainder of the high-methane gas from the CO scrubbers of 30 is supplied, at a rate of 31.0 standard cubic meters per hour, through a pressure booster 35 to a saturator 36. In this saturator, the gas is saturated with water vapor by being contacted with trickling hot water from the cooler 28 (heated cooling water), and the gas is then heated to 300 C. in the heat exchanger 21 before it is recycled to the gasification reactor 10.

What is claimed is:

1. In a process for converting methanol into a highmethane gas by cracking methanol vapor in one or more stages under superatmospheric pressure at elevated temperatures in contact with a catalyst, preferably in the presence of water vapor, the improvement which comprises adding a methanecontaining gas having a higher methane concentration than the gas produced by the cracking of methanol alone to the feedstock to be converted.

2. Process of claim 1 wherein said methanecontaining gas contains about 100% methane by volume.

3. Process of claim 2 wherein said methanecontaining gas contains more than methane by volume.

4. Process of claim 1 wherein the gas produced is passed through at least one methanation stage and then through a carbon dioxide scrubber and a partial stream of the methanized gas substantially free of carbon dioxide is recycled to the methanol-cracking stage.

5. Process of claim 1 wherein the gas produced is passed through at least one methanation stage and then through a cooler and a carbon dioxide scrubber and a partial stream of the gas thus treated is saturated in a saturator with condensate produced in the cooler, particularly with water vapor, and is recycled to the methanol-cracking stage.

6. Process of claim 1 wherien the weight ratio of water vapor to methanol in the gas mixture flowing in contact with the methanol-cracking catalyst is at least 0.7: 1.

7. Process of claim 1 wherein the weight ratio of the fresh water vapor added from the outside to methanol feedstock is below 4:1 and preferably below 2:1.

8. Process of claim 1 wherein a nickel catalyst containing 25-75%, preferably 35-60%, nickel by weight is used for the conversion of methanol.

9. Process of claim 8 wherein the maximum particle size of the nickel in the fresh catalyst material is 300 Angstroms and the most frequent particle size is in the range of 50-150 Angstroms.

10. Process of claim 8 wherein the fresh catalyst has a hydrogen adsorption above 3 milliliters per gram.

11. Process of claim 1 wherein the methanol is converted at a temperature of 250550C. and preferably at a temperature of 280500C.

12. Process of claim 1 wherein the pressure in the conversion reactor is in the range of 5-100, preferably l5-60, kilograms per square centimeter absolute pres- 

1. IN A PROCESS FOR CONVERTING METHANOL INTO A HIGH-METHANE GAS BY CRACKING METHANOL VAPOR IN ONE OR MORE STAGES UNDER SUPERATMOSPHERIC PRESSURE AT ELEVATED TEMPERATURES IN CONTACT WITH A CATALYST, PREFERABLY IN THE PRESENCE OF WATER VAPOR, THE IMPROVEMENT WHICH COMPRISES ADDING A METHANE CONTAINING GAS HAVING A HIGHER METHANE CONCENTRATION THAN THE GAS PRODUCED BY THE CRACKING OF METHANOL ALONE TO THE FEEDSTOCK TO BE CONVERTED.
 2. Process of claim 1 wherein said methane-containing gas contains about 75-100% methane by volume.
 3. Process of claim 2 wherein said methane-containing gas contains more than 90% methane by volume.
 4. Process of claim 1 wherein the gas produced is passed through at least one methanation stage and then through a carbon dioxide scrubber and a partial stream of the methanized gas substantially free of carbon dioxide is recycled to the methanol-cracking stage.
 5. Process of claim 1 wherein the gas produced is passed through at least one methanation stage and then through a cooler and a carbon dioxide scrubber and a partial stream of the gas thus treated is saturated in a saturator with condensate produced in the cooler, particularly with water vapor, and is recycled to the methanol-cracking stage.
 6. Process of claim 1 wherien the weight ratio of water vapor to methanol in the gas mixture flowing in contact with the methanol-cracking catalyst is at least 0.7:1.
 7. Process of claim 1 wherein the weight ratio of the fresh water vapor added from the outside to methanol feedstock is below 4:1 and preferably below 2:1.
 8. Process of claim 1 wherein a nickel catalyst containing 25-75%, preferably 35-60%, nickel by weight is used for the conversion of methanol.
 9. Process of claim 8 wherein the maximum particle size of the nickel in the fresh catalyst material is 300 Angstroms and the most frequent particle size is in the range of 50-150 Angstroms.
 10. Process of claim 8 wherein the fresh catalyst has a hydrogen adsorption above 3 milliliters per gram.
 11. Process of claim 1 wherein the methanol is converted at a temperature of 250*-550*C. and preferably at a temperature of 280*-500*C.
 12. Process of claim 1 wherein the pressure in the conversion reactor is in the range of 5-100, preferably 15-60, kilograms per square centimeter absolute pressure. 